Resonant acoustic structure for measuring well or borehole depth

ABSTRACT

A method and apparatus to measure the fluid depth in a well or borehole is described. The approach reduces the effects of acoustic noise occurring near the top of the well or borehole that can interfere with the fluid depth measurement. A resonant acoustic structure between an acoustic transducer and the well or borehole provides efficient coupling of spectrally narrow acoustic signals into the well or borehole, as well as providing a bandpass acoustic filter on the returning signal, to improve the signal to noise ratio of the desired acoustic reflection.

BACKGROUND OF THE INVENTION Field of Invention

The invention relates to determining the fluid depth in a well orborehole by measuring the time required for an acoustic event generatedat the top of the well or borehole to travel down the well or borehole,reflect from the fluid surface, and return to the top of the well orborehole. In particular, the invention relates to methods to improve thedetection of the reflected signal in the presence of noise thatinterferes with the fluid depth measurement.

Description of Related Art

It has become critical to collect information about the liquid level inwells or boreholes for a variety of reasons. These may include theability to manage water resources, monitoring civil engineeringstructures such as dams or buildings, and various earthworks such asbridges, roads, landfills, etc. It is important to determine the actualfluid level and have the ability to monitor fluid level changes overtime.

A number of techniques have been invented and commercialized over manydecades. One method involves introducing a sound pulse at the top of thewell or borehole and directing it to the fluid surface at the bottom ofthe well or borehole. By measuring the Time Of Flight (TOF) betweenlaunching the pulse and detecting the pulse that reflects from the fluidsurface, and knowing the speed of sound in the well or borehole, thedepth of the fluid surface can be estimated. A number of techniques haveextended this approach. For example, U.S. Pat. No. 4,934,186 issued Jun.19, 1990 discloses the detection of reflections from known, regularlyspaced collars along the well or borehole to provide calibration signalsfor the TOF measurement to the fluid surface. In U.S. Pat. No. 4,389,164issued Jun. 21, 1983 the inventors disclose the use of a TOF acousticpulse measurement system to control a pump and thereby maintain adesired fluid level in the well or borehole. U.S. Pat. No. 4,318,298issued Mar. 9, 1982 uses an acoustic TOF system to monitor the fluidlevel in a well or borehole on a periodic basis.

This approach suffers from false reflections that can result from aplurality of sources, including protrusions, changes in bore diameter,abrupt changes in well or borehole direction, changes in well orborehole wall composition, and resonant effects that can occur betweenone or more of the above-mentioned perturbations. For larger diameterwells or boreholes, a common mechanical structure at the top of the wellor borehole provides one or more side-mounted access ports into the mainhole. While the structure can provide convenient access for a depthmeasurement apparatus, it suffers from several drawbacks, including poorcoupling efficiency of acoustic power between the well or borehole andthe measurement apparatus, unwanted coupling of mechanical vibrationsgenerated by pumps placed in or near the well or borehole, and a lack ofdirect line of sight down the well or borehole, resulting in additionalstray reflections or attenuation of acoustic pulses that are otherwiseintended to travel down the well or borehole.

BRIEF SUMMARY OF THE INVENTION

An objective of the present invention is to provide an improved systemsuitable for measuring the level of water in a well or borehole, well orother environment and which substantially reduces the disadvantages ofearlier methods. Briefly, the invention consists of deploying a slendertube or pipe between an acoustic transducer assembly and the top of awell or borehole. The slender tube or pipe acts as an acoustic filterand an acoustic waveguide. The open ends of the tube act as acousticpartial mirrors, resulting in a series of resonant frequencies thatdepend on the length of the slender tube or pipe. When the acoustictransducer's output signal contains a substantial amount of energy in afrequency range that overlaps a natural resonant frequency of theslender tube or pipe, then a much larger acoustic signal is transferredinto the well or borehole than would otherwise occur. Upon reflectionfrom a liquid surface, the returning acoustic pulses are coupled intothe same slender tube or pipe, which transfers the acoustic pulses backto the transducer assembly, where they are detected using a microphoneor equivalent acoustic sensor. Since the reflected acoustic signalcontains a range of frequencies that match the resonant frequency of theslender tube or pipe, the reflected signal is efficiently coupledthrough the slender tube or pipe to a detector located near the acoustictransducer. In this way, the slender tube or pipe acts as a narrow bandacoustic filter, effectively blocking acoustic noise that can interferewith the detection of the acoustic reflection returning from the bottomof the well or borehole.

One advantage of the present invention is that extraneous acousticsignals that interfere with the distance measurement process arereduced.

Another advantage of the present invention is that it provides efficientcoupling of acoustic energy from a side-mounted transducer assembly intothe well or borehole.

Another advantage of the present invention is that it provides efficientcoupling of acoustic energy returning from a well or borehole, into aside-mounted transducer assembly.

Another advantage of the present invention is that it allows an acoustictransducer assembly to be easily retrofitted to an existing well orborehole installation.

Another advantage of the present invention is that it provides a meansof generating low frequency acoustic signals with substantial pressureamplitudes using a transducer with reduced dimensions.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail by reference to the includeddrawings, in which:

FIG. 1 illustrates a first acoustic measurement apparatus for measuringliquid depth in a well or borehole.

FIG. 2 illustrates a second acoustic measurement apparatus for measuringliquid depth in a well or borehole.

FIG. 3 is an illustration of an acoustic waveguide tube with open endsthat terminate into large diameter pipes most generally having differentdiameters.

FIG. 4 is an illustration of the acoustic transmission through anopen-ended tube versus the frequency of the acoustic signal.

FIG. 5 illustrates a third acoustic measurement apparatus for measuringliquid depth in a well or borehole.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a well or borehole 1 contains liquid 8 at adistance 7 from the well or borehole entrance. A housing 6 is fastenedto the well or borehole entrance and contains the components of thesensing apparatus. It is desired to determine the distance 7 bymeasuring the time of flight of an acoustic pulse generated by atransducer 3 that travels down the well or borehole 1, reflects off theliquid surface, and travels back up the well or borehole 1 to anacoustic detector 4. It is understood that the pulse generatingtransducer 3 and the detector 4 may be the same transducer in someembodiments of the present invention. In normal operation, theelectronic control unit (not shown) generates an electrical pulse thatresults in an acoustic pulse exiting transducer 3. The acoustic signalsdetected by detector 4 are processed by an electronic control unit (notshown) and further analyzed to create a measurement of the distance 7.In many well or borehole installations, it is common to include in thewell or borehole entrance assembly, one or more side access ports 2 thatenable the introduction of devices into the well or borehole withouthindering the operation of the well or borehole. In the diagram, ahousing 5 is attached to the access port 2 and contains the acoustictransducer 3 and acoustic detector 4. However, the acoustic signalsgenerated in housing 5 may experience a tortuous path before travelingdown the well or borehole 1, and reflections returning up said well orborehole experience the same tortuous pathways before arriving at theacoustic detector 4. In order to reduce the acoustic losses associatedwith these pathways, an acoustic waveguide tube 10 is introduced intothe access port. The acoustic waveguide tube 10 consists of a hollow,semi-flexible tube that is fastened at one end within the acoustichousing 5. The other end of the acoustic waveguide tube is positioned insuch a manner that its open end is approximately centered in the top ofthe well or borehole. In this way, the acoustic pulse generated by thetransducer 3 is coupled through the acoustic waveguide tube 10, exitsthe end of the waveguide tube, travels down and back inside the well orborehole, a portion of which is then collected by the waveguide tube 10,and is guided back to the acoustic detector 4 in the housing 5.

In FIG. 2, a second embodiment of the present invention is illustrated.The acoustic waveguide tube 10 has been eliminated. In this case, thesensor housing 5 is attached to an extension pipe 9, which is thenattached to the access port 2 at the well or borehole head housing 6.The acoustic transducer 3 generates an acoustic pulse that is guided bythe extension pipe 9. The acoustic reflection returning from the surfaceof water 8 is guided back to the detector 4 located inside the sensorenclosure 5.

In both of the above cases, it is difficult to couple acoustic energyfrom the transducer 3 into the waveguide 10 or the extension pipe 9, andalso difficult to couple acoustic energy from the waveguide 10 orextension pipe 9 into the well or borehole 1. This is important in that,for a fixed pressure pulse generated by the transducer 3, the reflectedacoustic signal pressure amplitude is reduced as the diameter of thewell or borehole 1 is increased. The present invention provides a meansof improving the coupling of acoustic energy from the transducer 3 intothe well or borehole 1, and from the well or borehole 1 back to thedetector 4.

The disclosed invention can be further understood with the aid of FIG.3, which illustrates the situation when an acoustic waveguide is used tocouple an acoustic signal between two larger regions. An acousticwaveguide 13 with open ends is positioned so that each end is locatedinside a larger pipe 11 and 12. The acoustic waveguide 13 has an insidediameter of d1. The two pipes 11 and 12 have larger diameter d2.Although the two pipes 11 and 12 shown in FIG. 3 have the same diameter,this is not required, and has only minor effect on the followingdiscussion.

The acoustic impedance in a region of wave propagation can be calculatedbased on the acoustic medium and the geometry of the region. Theacoustic power transmitted and reflected at the interface between tworegions with differing impedances can then be calculated to determinethe acoustic power transfer efficiency. The impedance of a region insidea tube where the acoustic wavelength is long compared with the diameterof the tube is given by

Z=ρc/S  (Equation 1)

where ρ is the mass density of the medium in the tube, c is the speed ofsound in the medium, and S is the cross-sectional area of the structurein which the acoustic wave is propagating, said area S being theprojection normal to the direction of propagation. Provided the acousticmedium is the same throughout the assembly, the ratio m of impedancesbetween two regions 1 and 2, defined as

$\begin{matrix}{m = {\frac{Z_{2}}{Z_{1}} = {\frac{\rho \; {c/S_{2}}}{\rho \; {c/S_{1}}} = \frac{S_{1}}{S_{2}}}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

can be used to calculate the reflected R and transmitted T acousticpowers as

$\begin{matrix}{R = \left( \frac{m - 1}{m + 1} \right)^{2}} & \left( {{Equation}\mspace{14mu} 3} \right) \\{T = \frac{4\; m}{\left( {m + 1} \right)^{2}}} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$

The larger the difference of impedances in the two regions, the largerwill be the resulting reflected power, and the lower will be theacoustic power that enters region 2. This effect is already significanteven for values of m=10 (or 0.1, depending on the direction of travel ofthe incident acoustic wave). For a circular cross-section tube of radiusr where S=πr², a value of m=10 corresponds to a diameter ratio of afactor of 3.3 In either case of m=10 or m=0.1, the reflected power isR=0.67, and the transmitted power is only T=0.33, or 33% of the incidentpower. This represents a significant loss of acoustic power. It alsoresults in a reflected acoustic wave that can interfere with thedetection of the desired acoustic signal returning from the liquid inthe well or borehole.

An acoustic wave already propagating in pipe 11 is indicated by thedirection of travel arrow 14. When this incident wave encounters the endof the acoustic waveguide 13, only a small fraction of the acousticpower is coupled into the waveguide 13. After propagating through thewaveguide 13, only a small fraction of the acoustic power is thencoupled into the larger pipe 12, shown as the arrow 16. The acousticpower coupled from pipe 11 into pipe 12 via waveguide 13 is givenapproximately by (1−R)², where R (shown by Equation 3) is the reflectioncoefficient at each end of the waveguide 13. Using the above example ofa pipe diameter that is approximately 3.3 times larger than thewaveguide diameter, the resulting acoustic power coupling efficiency isonly 11%. In the present application as a well or borehole depth sensor,the same coupling efficiency applies to the acoustic reflectionreturning from the well or borehole 1. The resulting round-trip acousticcoupling efficiency is only about 1%. This places difficult requirementson the sensitivity of the detector 4, and makes the measurementsusceptible to external sources of acoustic noise associated with waterpumps commonly found at well or boreholes of this type.

The present invention discloses a method to overcome this challenge. Animplicit assumption in the analysis just provided is that the length ofthe waveguide 13 is extremely long compared with the acoustic wavelengthin use. This leads to the assumption that interferences from reflectionsinside the waveguide 13 are not considered. In all realistic reductionsto practice, however, the several reflections inside the waveguide 13must be accounted for when predicting the transmission of acoustic wavesfrom region 11 to region 12. The general expression is given by (seeFahy, F., Foundation of Engineering Acoustics, 2001 Elsevier, pp202-204);

$\begin{matrix}{T_{waveguide} = \frac{4}{{4\; {\cos^{2}({kL})}} + {\left( {m + {1/m}} \right)^{2}{\sin^{2}({kL})}}}} & {{Equation}\mspace{14mu} (5)}\end{matrix}$

where T_(waveguide) is the transmission coefficient of the waveguide 13from region 11 to region 12, L is the length of the waveguide 13, and kis the acoustic wavenumber given by

$\begin{matrix}{k = \frac{2\; \pi \; f}{c}} & {{Equation}\mspace{14mu} (6)}\end{matrix}$

with f being the acoustic frequency.

The waveguide 13 behaves as a frequency selective filter, as illustratedby the graph shown in FIG. 4. When the acoustic frequency f is selectedso that an integer number of half-wavelengths can exist between the twoends of the waveguide 13, then constructive interference can occurwithin the waveguide, resulting in a higher acoustic amplitude exitinginto region 12 than would otherwise be expected. When this resonantcondition is not met, then the transmission through the waveguide 13 isvery low. As an illustrative example, a waveguide having a length L=1.5meters and a diameter d1 much smaller than the acoustic wavelength, issituated between two regions with larger diameters d2. Three curves ofthe transmission versus frequency are plotted for three different valuesof reflection R=0.0025, R=0.04 and R=0.67. These three values areachieved by using different diameters d2. As can be seen in the graph, alow reflection coefficient of R=0.0025 results in almost 100%transmission of the acoustic power from region 11 to region 12. Thiscorresponds to the condition where the diameters d1 and d2 are verynearly equal. As the reflection coefficient increases to R=0.04, thefrequency dependence of the transmission becomes more pronounced. At avalue of R=0.67, there remain certain resonant frequencies where theacoustic transmission is almost 100%, even though the transmission isvery low for non-resonant frequencies. In effect, the resonanceproperties of the waveguide 13 trade off transmission bandwidth forhigher throughput in a narrow range of frequencies.

This property is highly beneficial for the present application. Sincethe depth measurement technique can advantageously employ a narrow rangeof frequencies, the waveguide 13 provides a means of efficientlytransferring acoustic power from region 11 to region 12 by a suitableselection of operating frequencies, or in the case of a preferred, fixedoperating frequency, by a suitable selection of length L of waveguide13.

A second benefit of the deployment of a resonant waveguide 13 is due tothe reciprocal acoustic properties of the waveguide. For acousticreflections returning from the well back to the detector, the acousticfrequencies present in the reflected signal will be substantiallysimilar to those contained in the outgoing acoustic signal. Because ofthis, the returning acoustic signal will be efficiently transferred fromregion 12 back to region 11, resulting in an increased signal amplitudeat the detector 4.

A third benefit of the deployment of a resonant waveguide 13 is due tothe frequency selectivity of the waveguide. In practical application ofthe present invention, there are often found sources of acoustic noisefrom ancillary equipment such as pumps associated with the well site.The frequency spectrum of these acoustic noise sources tends to bebroadband, and they tend to occur at frequencies higher than thoseadvantageously employed in a well depth measurement. It is desired thatthese interfering noise sources be attenuated as much as possible at thedetector 4 to minimize the need for further electronic filtering of thedetected signal. The resonant waveguide 13 is a passive frequency filterthat efficiently transfers the frequency components of the desiredsignal returning from the well, while at the same time attenuating manyof the frequency components of the undesired noise being generated byancillary equipment at the well.

A fourth benefit of the deployment of a resonant waveguide 13 is due tothe resonant frequency's dependence on the length L of the waveguide 13.Well depth measurements in some cases make use of low frequency acousticpulses and suitable interpolation techniques applied to the returningsignal to perform accurate depth measurements. The low frequenciesselected result in improved depth detection reliability. However, itbecomes difficult to generate low frequency acoustic signals withappreciable amplitude while using a physically compact acoustictransducer. The resonant acoustic waveguide 13 solves this challenge bydefining the acoustic resonance condition based on the length of thewaveguide L. In effect, the waveguide 13 extends the resonant cavitydimension for the acoustic transducer, lowering the resonance frequencywhere efficient generation of acoustic energy can take place. This isparticularly helpful when applying the depth measurement to wells withlarger diameter boreholes, such as greater than 50 cm (12 inches) indiameter, that are fitted with access ports having diameters of 2.5 cmto 5 cm (1 to 2 inches) at most.

Although the predicted operation of the resonant waveguide includes anassumption of a continuous wave acoustic signal, in practicalapplications the acoustic signal is comprised of one or more half-cyclesof an acoustic sinusoidal wave, in the form of a tone burst. In thiscase, the resonance behavior of the waveguide 13 is modified relative tothat shown in FIG. 4. The depth of modulation of the transmission willbe less than what can be achieved with continuous wave signals. However,all of the benefits listed above remain in the case of using an acoustictone burst, including the attenuation of undesired noise reaching thedetector 4.

When the ends of the acoustic waveguide are both open and have non-zeroreflection coefficients, as previously disclosed, the resonantfrequencies f_(oo) (where the subscript refers to the waveguide havingboth ends open) are given by

$\begin{matrix}{f_{oo} = \frac{nc}{2\left( {L + {0.8\; d}} \right)}} & \left( {{Equation}\mspace{14mu} 7} \right)\end{matrix}$

where the integer n =1, 2, 3 . . . and d is the diameter of the acousticwaveguide. In normal operation, the first resonant frequency is selectedto achieve the lowest possible operating frequency for a given length ofacoustic waveguide L.

A third embodiment of the disclosed invention is shown in FIG. 5. Inthis case, the acoustic transducer 3 forms one sealed end of the pipe 9.In this case, the acoustic signal is coupled through the pipe 9 into thewell 6. An acoustic reflection occurs at the junction between the sideport 2 and the well 6. By selecting the acoustic frequency to beresonant with the combined length of the pipe 9 and the side port 2, theresulting acoustic power coupled into the well 6 is greatly increased.All of the advantages described for the previous embodiment also applyto this arrangement. The only difference is a shift in the resonantfrequencies. According to acoustic analysis of ideal acoustic waveguideshaving one closed end and one open end, the resonant frequencies f_(co)(where the subscript refers to the waveguide having one end closed andthe other end open) are given approximately by the following equation:

$\begin{matrix}{f_{co} = \frac{\left( {{2n} - 1} \right)c}{4\left( {L + {0.4\; d}} \right)}} & \left( {{Equation}\mspace{14mu} 8} \right)\end{matrix}$

where the integer n=1, 2, 3 . . . This differs from the resonantfrequencies predicted for the acoustic waveguide previously described,and can be more clearly understood by way of an example. For example,with a length of L=1.5 meters, c=340 meters/second, and d=0.1 meters,the first resonant frequencies are f_(oo)=107 Hertz and f_(co)=55 Hertz.When a low resonant frequency is desired, the second embodiment ispreferred when the length of the acoustic waveguide L is restricted byother factors.

In most practical implementations of the disclosed invention, thedimensions of the acoustic guiding structure (either a waveguide tube 10or extension pipe 9) are comparable to the dimensions of the sensorhousing 6. In these cases, the housing must be considered a part of theoverall acoustic resonant structure for the purposes of calculating theresonant frequencies of the complete structure. The housing 6 behaves asa Helmholtz resonant cavity that is mechanically fastened (andacoustically coupled) to an acoustic waveguide tube 10 or extension pipe9. The strength of the acoustic coupling between the acoustic waveguideand the housing 6 is determined by the diameter of the acousticwaveguide tube 10 or extension pipe 9 relative to the diameter of thehousing 6, This transition results in an acoustic reflection thatcontributes to the overall acoustic behavior of the structure. As aresult, the resonant frequencies of the complete structure will be lowerthan those calculated by ignoring the housing 6. An approximation forthe resonant frequency f_(r) is given by (see Fahy, F., Foundation ofEngineering Acoustics, 2001 Elsevier, pp 60-61);

$\begin{matrix}{f_{r} = {\frac{c}{2\; \pi}\sqrt{\frac{S}{V\left( {L + L^{\prime}} \right)}}}} & \left( {{Equation}\mspace{14mu} 9} \right)\end{matrix}$

where S is the cross sectional area of the waveguide tube 10 orextension pipe 9, V is the volume of the housing 6, L is the length ofthe waveguide tube 10 or extension pipe 9, and L′ is a correction to thelength L to account for the difference in diameters between the housing6 and the extension pipe 9 or waveguide tube 10. An important result isthat the lowest resonant frequency of the structure depends on thediameter as well as the length of the waveguide tube 10 or extensionpipe 9, in addition to the volume of the housing 6. Although thiscomplicates the prediction of the resonant frequencies, the functionalfeatures of the disclosed apparatus retain the already listed benefitsto measuring the depth of a well or borehole. Since every installationon a well or borehole will have unique physical features that precludethe ability to precisely predict the resonant frequency for operation, atechnique for automatically determining the resonant frequency should beincluded as part of the well or borehole measurement system to achievethe best possible performance.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is understood that the appended claims areintended to cover all such modifications and changes as fall within thetrue spirit of the invention.

What is claimed is:
 1. An apparatus for measuring the distance to anacoustically reflective surface in a well or borehole, comprised of: asubstantially airtight housing containing an acoustic transducer togenerate an acoustic signal and an acoustic detector to measure acousticsignals; a pipe having a diameter substantially smaller than thediameter of said housing, and fastened over an aperture in said housing;where said pipe fastened to an access port of a well or borehole, wheresaid well or borehole has a diameter substantially larger than saidpipe; where said acoustic generator produces an acoustic signal with asubstantial fraction of energy in a range of frequencies that coincidewith at least one resonant frequency of said pipe.
 2. The apparatus inclaim [1] where the transducer is selected from the list including butnot limited to: an electromagnetic speaker, a piezoelectric disc, anelectrostatic speaker, and a compressed air solenoid.
 3. The apparatusin claim [1] where the pipe has a circular cross-section with a diameterin the range of 1 cm to 30 cm.
 4. The apparatus in claim [1] where thepipe is comprised of a material selected from the list including but notlimited to steel, cast iron, polyvinyl chloride, copper, brass, nylon,and fluorinated polymer, polyethylene or stainless steel.
 5. Theapparatus in claim [1] where the acoustic detector is selected from thelist including but not limited to a condenser microphone, an electretmicrophone, a micro-machined microphone, a piezoelectric microphone, anelectromagnetic microphone, or a speaker.
 6. The apparatus in claim [1]where the well or borehole has a diameter in the range of 1 cm to 200cm.
 7. The apparatus in claim [1] where the length of the pipe is in therange of 1 cm to 1000 cm.
 8. The apparatus in claim [1] where theacoustic resonant frequency is in the range of 1 Hertz to 500 Hertz. 9.(canceled)
 10. The apparatus in claim [1] where the acoustic signalgenerated by the acoustic transducer has the form of a tone burst, wherethe number of cycles in the tone burst is selected to be in the range of0.1 to
 100. 11. (canceled)
 12. The apparatus in claim [1] where theacoustic signal generated by the acoustic transducer has the form of atone burst, where the tone burst is repeated with a time intervalselected between 0.1 second and 1000 seconds.
 13. An apparatus formeasuring the distance to an acoustically reflective surface in a wellor borehole, comprised of: a substantially airtight housing containingan acoustic transducer to generate an acoustic signal and an acousticdetector to measure acoustic signals; a tube having a diametersubstantially smaller than the diameter of said housing, fastened overan aperture in said housing; where said tube is contained within a pipehaving a diameter substantially larger than said tube but substantiallysmaller than said housing; where said pipe is fastened over an accessport of a well or borehole, where said well or borehole has a diametersubstantially larger than said pipe; where said tube passes through saidaccess port of said well or borehole and terminates inside said well orborehole; where said acoustic generator produces an acoustic signal witha substantial fraction of energy in a range of frequencies that coincidewith at least one resonant frequency of said tube.
 14. The apparatus inclaim [13] where the transducer is selected from the list including butnot limited to: an electromagnetic speaker, a piezoelectric disc, anelectrostatic speaker, and a compressed air solenoid.
 15. The apparatusin claim [13] where the pipe has a circular cross-section with adiameter in the range of 1 cm to 30 cm.
 16. The apparatus in claim [13]where the pipe is comprised of a material selected from the listincluding but not limited to steel, cast iron, polyvinyl chloride,copper, brass, nylon, and fluorinated polymer, polyethylene or stainlesssteel.
 17. The apparatus in claim [13] where the acoustic detector isselected from the list including but not limited to a condensermicrophone, an electret microphone, a micro-machined microphone, apiezoelectric microphone, an electromagnetic microphone, or a speaker.18. The apparatus in claim [13] where the well or borehole has adiameter in the range of 1 cm to 200 cm.
 19. The apparatus in claim [13]where the length of the pipe is in the range of 1 cm to 1000 cm.
 20. Theapparatus in claim [13] where the acoustic resonant frequency is in therange of 1 Hertz to 500 Hertz.
 21. (canceled)
 22. The apparatus in claim[13] where the acoustic signal generated by the acoustic transducer hasthe form of a tone burst, where the number of cycles in the tone burstis selected to be in the range of 0.1 to
 100. 23. (canceled)
 24. Theapparatus in claim [13] where the acoustic signal generated by theacoustic transducer has the form of a tone burst, where the tone burstis repeated with a time interval selected between 0.1 second and 1000seconds.